An unenforced law (also symbolic law) is a crime which is illegal, but is usually not penalized by a jurisdiction. It is a law which is usually ignored by law enforcement, therefore such laws have no consequences. Some people have criticized such laws claiming that it indicates sometimes the law means what it says, sometimes it doesn't.
Overview
The law may be put in place simply for symbolic reasons, but without actual prosecutions taking place. For example speeding is illegal, but patrol officers usually ignore motorists who are 5-10 miles an hour over the legal limit. Automate ticketing systems like computerized cameras will still issue fines in these circumstances in some but not all US states.
Although incest is illegal in many European countries, it is generally not enforced if between two consenting adults.
Laws which are symbolic typically attempt to persuade rather than enforce, punish or prevent. An example is adultery in Colorado, where it is prohibited by law, but not punishable in any way.